When main power to an elevator system is lost, power to the elevator hoist motor and the emergency brake associated with an elevator car is interrupted. This causes the hoist motor to stop driving the car, and causes the emergency brake (which is disengaged when energized) to drop into engagement with the drive shaft. As a result, the car is stopped almost immediately. Because the stopping may occur randomly at any location within the elevator hoistway, passengers may be trapped in the elevator car between floors. In conventional systems, passengers trapped in an elevator car between floors may have to wait until a maintenance worker is able to release the brake and control cab movement upward or downward to allow the elevator car to move to the nearest floor. It may take some time before a maintenance worker arrives and is able to perform the rescue operation.
Elevator systems employing automatic rescue operations (ARO) have been developed. These elevator systems include a backup electrical power source that is controlled after a main power failure to provide backup power to move the elevator car to the next floor landing. Conventional automatic rescue operation systems typically use a battery as the backup emergency power source. They attempt to direct the rescue run into the “light” direction, i.e., the direction that gravity will tend to move the car as a result of weight difference between the car with its passengers and the counterweight. The automatic rescue system makes use of load weighing devices to determine the “light” direction. The hold current is applied to the hoist motor to apply a torque in a direction opposite to the load imbalance sensed by the load weighing device, so that the elevator car will not move while the brake is being lifted. Once the brake has been lifted, the system attempts to drive the car in the light direction, as indicated by signals from a load weighing device. The battery as well as the supply circuitry must be dimensioned to deliver a peak hold current for a maximum load in the car.
In some cases, the determination of the light direction may be difficult using load weighing devices. If the light direction is determined incorrectly because load weighing has failed, or the load weighing signals have been misinterpreted, an attempt could be made to drive the car in the heavy direction. This can result in larger peak currents and in increased energy consumption.
The automatic rescue operation system must account for an energy reserve, and require failure handling logic in case the load weighing has failed and a run is attempted into the “heavy” direction. The peak current and energy capacity required for the start phase, and for the failure scenario in which a run in the “heavy” direction is attempted, significantly exceed the requirements for moving a balanced load or for operating the elevator once the start phase has passed and the elevator is moving in the “light” direction.